Wide field deep sky astrophotography using camera lenses

My experiments using camera lenses for wide field deep sky astrophotography are coming to a close.  I am not satisfied with the quality of images acquired.  The main issue I have found is that stopping down the lenses to overcome optical aberrations and obtain useable stars reduces clear aperture which significantly affects the ability of the lens to gather enough light for extended faint objects. 

The wide field rig I have been using is shown above.  It is based on a QSI 583wsg mono CCD with lens adapter for Pentax 6x7 medium format lenses.  The rig uses standard ADM mounting hardware (dovetails and guide scope rings) and a custom bracket manufactured by Lakeside Astro to support their ASCOM compliant motorised focussing system.  The stepper motor drives a timing belt which goes around the focus ring of the lens allowing automatic focussing during an imaging session.  The rig worked well using my standard imaging software stack of MaxIm DL and FocusMax under ACP control.  Good V-curves for focussing were obtained for different lenses and produced accurate and repeatable results.

Two vintage SMC Pentax 6x7 lenses were tested - a 165mm F/2.8 and a 300mm F/4.  Mint condition 1980’s versions of these lenses were purchased second hand from Japan for around £100 each.  To produce useable stars both lenses were stopped down by two stops from wide open.  This significantly reduced clear aperture for the 300mm lens and coma was still present in the corners of images.  Higher quality results were obtained using the 165mm lens.  My conclusion is that for deep sky work it would be worth spending a bit more and purchasing a good quality short focal length apochromatic refractor with ED glass. 

See Jerry Lodriguss (Catching the Light) for a good and balanced review of camera lenses for astrophotography.  High quality camera lenses are well suited for landscape astrophotography and there are numerous articles on the web on choosing and using lens for Milky Way photography.